In semiconductor manufacture, a need exists to achieve higher component densities and smaller, e.g. nanoscale, structures in integrated circuits and nanoelectromechanical systems. The lithography techniques to produce semiconductor devices typically involve applying patterns of the device structure onto a resist layer, and selectively etching away the substrate exposed by the pattern in the resist layer. In further processing steps, other materials may be deposited in the etched areas to form, for example, an integrated circuit.
In conventional photolithography a pattern mask is projected by light onto a photo-sensitive polymer resist. However, the resolution of this approach is inherently limited by diffraction. Alternatively, a pattern may also be transferred to a suitable resist layer by energy waves of a different radiative quality, such as in electron beam lithography, ion beam lithography or x-ray lithography. However, with such lithography methods, producing nano-scale structures at an acceptable cost and industrially acceptable throughput remains difficult.
Directed self-assembly (DSA) of block-copolymers is an emergent alternative approach to nanolithography. Block-copolymers consist of chemically different polymer blocks interconnected by covalent bonds. The chemically different polymer blocks undergo a microphase separation, which is driven by repulsion between the dissimilar polymer chains, such that homogenous domains in a periodic nanostructure are formed after annealing. For example, such periodic structures may comprise hexagonally packed cylinders, spheres, gyroid structures or lammelae. The type of structure which is formed is furthermore controllable by tuning the ratios of the different polymer block lengths. However, the block-copolymer material may feature random orientation and a poor long-range order when not constrained by orientation control techniques. Such techniques, for example graphoepitaxy or chemical epitaxy, selectively direct the formation of domains in the block-copolymer material. Through subsequent selective removal of one polymer type, a patterned structure of gaps is formed which can be used as a resist layer on the underlying substrate, thus enabling feature patterning on scales in the range of 5 nm to 50 nm.
In DSA, a pre-pattern may be applied on the substrate to direct the orientation of the block-copolymer material when applied thereon. This pre-pattern may be used to achieve frequency multiplication, e.g. generating cross-bar structures or line patterns of higher spatial frequency than the pre-pattern, thus increasing the pitch of the final printed structure. Therefore, advanced scale-down patterning may be achieved by DSA with pattern features smaller than 14 nm. Moreover, DSA can be used to repair defects and improve uniformity in the original print, e.g. by shrinking and rectifying the pre-pattern. For example, in combination with EUV lithography, limitations imposed by local variations in the critical dimension (CD) may be overcome, which may be, for example, advantageous for manufacturing small contact features.
For selectively removing one polymer type in DSA by etching, an argon-oxygen (Ar/O2) plasma may be used according to DSA BCP etching methods known in the art. In order to achieve higher selectivity between the polymer components, e.g. to efficiently remove polymethylmethacrylate (PMMA) while leaving polystyrene structures substantially unaffected in a polystyrene-block-polymethylmethacrylate (PS-b-PMMA) block-copolymer it may be known to reduce the O2 concentration in Ar/O2 gas mixture. For example, For a substantially pure Ar plasma with no bias, a PS/PMMA selectivity up to 8 may be achieved. By introducing 10% O2 in the Ar/O2 plasma, the selectivity may reduce, for example, to 2. Unfortunately, it is also known that an argon plasma without added oxygen may not yield acceptable PMMA etching, for example as indicated in FIG. 4, which shows a photographic reproduction of a PS-b-PMMA BCP layer after argon etching.
It is known in the art to provide a protective covering layer, for example using CH3F or SiCl4, on top of the polystyrene to improve etching specificity. For example, in the International Patent Application No. WO 2012/031818, a first selective etching is used to reduce the thickness of one of the polymer domains, a planarization layer is deposited over the block-copolymer material, which is subsequently etched away such that caps remain over the polymer domain with reduced thickness. These caps protect the underlying material during a final etching phase.